1. Field of the Invention
This invention relates to a fixing apparatus to be used in electrophotographic copying device, printer, and facsimile. More particularly, the invention relates to an induction heating apparatus for fixing a toner image on a recording medium by means of induction heating.
2. Description of the Prior Art
The electrophotographic copying devices, printers, facsimiles, etc. are provided with a fixing apparatus for fixing a toner image on sheets such as recording paper and transfer material which are recording media. The fixing apparatus of this class, on receiving a sheet which has an unfixed toner image transferred in advance thereto, effects the fixation of the toner image on the sheet by causing the sheet to be nipped and conveyed between a fixing roller heated to a preset temperature and a pressing roller opposed to the fixing roller across a narrow gap thereby imparting the heat of the fixing roller to the toner and consequently melting the toner enough for the fixation.
This fixing apparatus uses, as the voltage to be supplied to a heat source for heating the fixing roller, the voltage of the commercial power supply (AC 100 V in the territory of Japan) in its unmodified form or as further elevated (above AC 100 V). It, therefore, requires to separate electrically a circuit for supplying electric power to the heat source and a control circuit system adapted to control the fixing apparatus as a whole and driven at a relatively low voltage. This measure is taken, in due respect of the fact that the control circuit system is connected to such parts as the connecting terminals and switches for various interfaces which are open for direct contact of the user's hands, to prevent the high voltage supplied to the heat source from flowing to the control circuit system by accident of some sort or other.
As respects the control of the temperature of this fixing apparatus, the conventional fixing apparatus which uses a halogen lamp as the heat source effects this control by monitoring the temperature of the fixing roller and turning off the halogen lamp whenever the detected temperature surpasses the prescribed level necessary for the fixation or turning on the halogen lamp after the detected temperature has fallen below this prescribed level thereby keeping the temperature of the fixing roller at the prescribed temperature range. This control is referred to as "ON/OFF" control.
In recent years, among other fixing apparatus, the fixing apparatus which utilizes induction heating has come to attract particular attention on account of the demand for quick start and economization of energy. The induction heating fixing apparatus is characterized by the quickness of temperature elevation because the fixing roller made of metal is caused to emit heat in itself by the magnetic flux generated by a high-frequency current flowing through a coil.
This induction heating fixing apparatus, however, incurs difficulty in keeping the temperature of the fixing roller at the prescribed level when it resorts to the ON/OFF control which is usually found in the conventional fixing apparatus using a halogen lamp.
The reason for this difficulty is as follows. In the fixing apparatus using a halogen lamp as the heat source, since the halogen lamp itself has a certain heat capacity besides the fixing roller inherently having a heat capacity and, therefore, is not immediately cooled after a power interruption and further since the fixing roller is indirectly heated by the thermal radiation from the halogen lamp and, therefore, the temperature of the fixing roller cannot be suddenly raised when the halogen lamp is turned on, the temperature can be kept at a relatively constant level even by the ON/OFF control. In contrast thereto, in the induction heating fixing apparatus, owing to the greatness of heating speed (temperature-increasing speed) which constitutes itself the salient characteristic of this apparatus, the fixing roller has the temperature thereof elevated in a very short time (several seconds) to the level necessary for the fixation (in the approximate range of 180 to 200.degree. C.) after the coil has received a high-frequency current from an inverter circuit and it has the temperature abruptly lowered after the switch has been turned off. The ON/OFF control which is effected in this manner, therefore, has the problem of incurring difficulty in attaining a constant temperature. Particularly the induction heating fixing apparatus which uses a metal sleeve flexible because of such a small wall thickness as falls in the approximate range of several .mu.m to some tens of .mu.m is difficultly feasible as a fixing implement because the ON/OFF control, owing to the small thermal capacity of the metal sleeve, is suffered to give rise to a temperature ripple as much as 30 to 40.degree. C., a level far exceeding the level of 4 to 5.degree. C. which is accepted as proper for the temperature ripple of a fixing apparatus.